The present invention relates to image sensors and, more particularly, to image sensors in which pixel signals can be selectively summed to increase low light signals.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Image sensors are formed from pixels. Pixels may be arranged in a two-dimensional array of rows and columns. The pixels contain photodiodes that convert incoming light into electrical charge. The amount of charge that is produced by each pixel is proportional to the amount of light received by that pixel. By measuring the amount of charge produced by each of the pixels in an image sensor, a digital image can be constructed.
Charge summing arrangements are sometimes used in image sensors to improve signal strength. When charge summing is enabled, the amount of charge that is produced by a number of pixels is added together. Because more than one pixel contributes to the signal, image sensor resolution is decreased. Nevertheless, the process of combining charge from more than one pixel increases the signal-to-noise ratio and thereby improves image quality at low light levels.
Image sensors typically include a charge storage node called a “floating diffusion.” Charge that has been produced by the photodiode in a pixel can be stored on the floating diffusion before being read out by readout circuitry in the sensor. In image sensors with charge summing capabilities, the charge from multiple photodiodes is summed on a common floating diffusion.
One figure of merit for image sensors is the so-called “conversion gain” of the sensors' pixels. Conversion gain is a measure of how efficiently voltage changes are produced on a floating diffusion in response to the production of charge on a photodiode. Conversion gain is typically defined as the number of volts produced per electron in a pixel (typically presented in units of μV/e−). Sensors that exhibit poor conversion gain tend to produce images that are of lower quality than sensors that exhibit high conversion gain.
Although it is often desirable to provide an image sensor with charge summing capabilities so that the image sensor can handle low-light imaging tasks, care should be taken to ensure satisfactory conversion gain. Conventional charge summing arrangements may add undesirable excess capacitance to the floating diffusion, which can adversely affect conversion gain.
It would therefore be desirable to provide image sensors with improved charge summing capabilities.